Common wiring substrates have a thermal expansion coefficient substantially matching that of copper wiring. While high heat-resistance wiring substrates according to FR4 (flame retardant: a symbol indicating the grade of flame resistance of a copper-clad laminate board serving as a member of a wiring substrate) have been commercialized in recent years, the wiring substrates generally have a thermal expansion coefficient of 15 ppm/° C. or more.
LSI (large scale integration) elements formed on a silicon wafer have a thermal expansion coefficient of 3 to 3.5 ppm/° C. Therefore, it is desirable that package substrates on which the LSI elements (bare chips) are directly mounted and wiring substrates for probe cards for testing the silicon wafer in a high-temperature or low-temperature circumstance have a thermal expansion coefficient matching that of the silicon wafer. In order to meet such desires, it is known to obtain a wiring substrate having a thermal expansion coefficient matching that of a silicon wafer by using a carbon base material obtained by impregnating carbon fibers having a low thermal expansion coefficient and a high elasticity modulus in a core portion of the wiring substrate, or by sandwiching a desired layer between carbon base materials.
FIG. 1 illustrates a typical cross-sectional structure of a wiring substrate (double-sided substrate) according to the related art in which a carbon base material is used. A wiring substrate 100 illustrated in FIG. 1 includes a base material 102 which is a carbon composite material, wiring layers 103 stacked on the front surface and the back surface of the base material 102, and wiring patterns 104 formed on the wiring layers 103. Further, two types of through hole vias 105A and 105B for electrical conduction between the upper and lower wiring layers 104 are formed to penetrate through the wiring layers 103 and the base material 102.
The through hole via 105B, which is illustrated on the right side of the drawing, is a single-structure through hole via for electrical conduction with the base material 102 which is conductive, and is used for connection with a power source. Meanwhile, the through hole via 105A, which is on the left side, is a double-structure through hole via surrounded by an insulating resin 106, and is electrically insulated from the base material 102. The double-structure through hole via 105A is used for connection with another power source separated from the power source for the base material 102, or for connection with a signal.
However, the carbon composite material is prepared by pressing a matrix resin having a high thermal expansion coefficient onto carbon fibers in the thickness direction, and therefore has a high thermal expansion coefficient in the thickness direction compared to that in the planar direction.
The following is reference document:    [Document 1] Japanese National Publication of International Patent Application No. 2009-544153